Magento is a strong ecommerce platform and has plugin features to enhance functionality. They are so many registered in Magento Connect but below are some of the plugins that I really like.

Canonical URL’s for Magento – This extension adds the new canonical links to the head of your Magento pages.
Yoast Blank SEO Theme – This is a clean simplistic SEO optimized Theme based on the core Magento Blank Theme. This theme can be easily customized by professional designers.
Auto CrossSell Products – This small extension attempts to automate the product cross sell maintenance. Every time a customer checks out, all products that are in the shopping cart are linked as cross sell products.
Fooman Invoice=Order Number creates separate invoice, shipment and credit memo numbers for one order.
Teaserbox shows the teasers of your products if they are available.
Lazzymonks Twitter – Displays Twitter updates in the menu and allows updates from within the admin panel
MailChimp ECommerce360 Reporting – The MailChimp Ecommerce Tracking Plugin helps you measure the ROI of your email campaigns. We’ll track when people click from your email campaigns, visit your website, all the way to purchase. Then we’ll aggregate the information and display it on your MailChimp campaign report.
Advanced Sitemap lists products from current store
Events Calendar displays your events on your website.